Plants are multicellular terrestrial and photosynthetic Multicellular Different

Plants are multicellular, terrestrial and photosynthetic Multicellular: Different cells can have various functions, but they must integrate their activities Photosynthetic: Plants and many other organisms can convert solar energy to chemical energy Terrestrial: Plant ancestors were aquatic, but terrestrial plants have to cope with very dry air Leaf cross section image from Bouton, J. H. , et al. , (1986). Photosynthesis, leaf anatomy, and morphology of progeny from hybrids between C 3 and C 3/C 4 Panicum Species. Plant Physiol. 80: 487 -492. © 2017 American Society of Plant Biologists

What are plants? Plants are photosynthetic eukaryotes (Circles not drawn to scale) Nonphotosynthetic bacteria Archaea ALL LIFE PHOTOSYNTHETIC ORGANISMS Green sulfur bacteria Purple sulfur bacteria Other bacteria Cyanobacteria EUKARYOTES WITH CYANOBACTERIA -DERIVED CHLOROPLASTS Diatoms Brown algae Fungi CYANOBACTERIA + “DESCENDANTS” Red algae GREEN ALGAE AND DESCENDANTS PLANTS Animals You are here © 2017 American Society of Plant Biologists

Plants descended from a eukaryotic ancestor + a cyanobacteria Bacteria Plastid endosymbiosis Archaea ANIMALS FUNGI PLANTS ALGAE Eukarya >0. 5 BYA >1. 5 BYA Photosynthesis evolved in bacteria. All photosynthetic eukaryotes acquired this ability through endosymbiosis of photosynthetic bacteria Therefore, some “plant” genes (those derived from the ancestral bacteria) are more like bacterial genes than the genes of other eukaryotes Mitochondrial endosymbiosis ORIGIN OF LIFE >3. 5 BYA Adapted from Govindjee and Shevela, D. (2011). Adventures with cyanobacteria: a personal perspective. Frontiers in Plant Science. 2: 28. © 2017 American Society of Plant Biologists

Plants are photosynthetic eukaryotes High energy, reduced carbon Energy input from sunlight Low energy, oxidized carbon in carbon dioxide Oxygen is released as a byproduct • Plants convert light energy to chemical energy • Photosynthesis evolved in bacteria, and takes place in the descendants of endosymbiotic photosynthetic bacteria • Through photosynthesis, plants and algae are responsible for the transfer of most of the energy that enters the biosphere 6 CO 2 + 6 H 2 O C 6 H 12 O 6 + 6 O 2 © 2017 American Society of Plant Biologists

Photosynthesis can be understood as two sets of connected reactions 2 NADPH e− 2 H+ 2 NADP+ 2 H 2 O O 2 + 2 H + + 2 e− ADP ATP Chloroplast H+ The LIGHT reactions take place in the thylakoid membranes The CARBON-FIXING reactions take place in the chloroplast stroma Adapted from Kramer, D. M. , and Evans, J. R. (2010). The importance of energy balance in improving photosynthetic productivity. Plant Physiol. 155: 70– 78. © 2017 American Society of Plant Biologists

Light harvesting reactions produce O 2, ATP and NADPH 2 NADPH The reactions require several large multi-protein complexes: two light harvesting photosystems (PSI and PSII), the cytochrome b 6 f complex, and ATP synthase Cytochrome b 6 f complex ADP ATP 2 H 2 O O 2 + 2 H + + 2 e− Photosystem II (PSII) e− 2 H+ 2 NADP+ Photosystem I (PSI) H+ ATP synthase Adapted from Kramer, D. M. , and Evans, J. R. (2010). The importance of energy balance in improving photosynthetic productivity. Plant Physiol. 155: 70– 78. © 2017 American Society of Plant Biologists

Chlorophyll captures light energy to initiate the light harvesting reactions First step of photochemistry Chl* Chlorin ring captures photons e- Photon 4 H+ Chlorophyll is held in pigmentprotein complexes in a highly organized manner Chl Photon capture by chlorophyll excites the chlorophyll (Chl*). Chl* can lose an electron to become oxidized chlorolphyll (Chl+) Chl e- 2 H 2 O O 2 Chl+ is reduced by stripping an electron from water, releasing oxygen and protons Buchanan, B. B. , Gruissem, W. and Jones, R. L. (2000) Biochemistry and Molecular Biology of Plants. American Society of Plant Physiologists. © 2017 American Society of Plant Biologists

Through the Calvin-Benson cycle, ATP and NADPH are used to fix CO 2 3 x Ribulose-1, 5 bisphosphate (Ru. BP) Each CO 2 fixed requires 3 ATP and 2 NADPH 3 x CO 2 Rubisco Carboxylation 6 ADP + 6 Pi Energy input 6 ATP Regeneration 6 x glyceraldehyde 3 -phosphate (GAP) 5 x GAP 6 x 3 -phosphoglycerate (3 PG) Reduction 6 ATP Energy input 6 ADP + 6 Pi For every 3 CO 2 fixed, one GAP is produced for biosynthesis and energy 1 x GAP 6 NADPH Reducing power input 6 NADP+ + 6 H+ Adapted from: Buchanan, B. B. , Gruissem, W. and Jones, R. L. (2000) Biochemistry and Molecular Biology of Plants. American Society of Plant Physiologists. © 2017 American Society of Plant Biologists

Plants are assembled from cells NUCLEUS CHLOROPLASTS MITOCHONDRIA EN DO Plasma membrane ME MB VACUOLE Vacuolar membrane (tonoplast) Plasma membrane Cell wall RA NE S The plasma membrane is a barrier that lets cells maintain a different internal environment from their surroundings A “typical” plant cell © 2017 American Society of Plant Biologists

Cells are surrounded by a semipermeable plasma membrane The membrane is permeable to water and gases, but impermeable to ions and larger molecules CO 2 OUT H 2 O Na+ K+ IN K+ Photo credit: Wenche Eikrem and Jahn Throndsen, University of Oslo © 2017 American Society of Plant Biologists

Proteins in the plasma and vacuolar membranes move molecules H+ H+ X X ATP ADP H+ VACUOLE These transporters bring in needed ions and other compounds, and export unwanted molecules. The transporters are also needed to regulate the cell’s osmotic potential H+ Vacuolar membrane Plasma membrane Cell wall Adapted from Hedrich, R. (2012). Ion channels in plants. Physiol. Rev. 92: 1777 -1811. © 2017 American Society of Plant Biologists

(Most) plant cells are connected by plasmodesmata Plasmodesmata are plasma-membrane lined, regulated cytoplasmic bridges between plant cells Signals, nutrients, ions and water can move to adjacent cells, or longer distances via the phloem Guard cells are isolated, without functional plasmodesmata Pathogens can spread through the plant through plasmodesmata Reprinted from Lee, J. -Y. and Lu, H. (2011). Plasmodesmata: the battleground against intruders. Trends Plant Sci. 16: 201 -210 with permission from Elsevier. © 2017 American Society of Plant Biologists

Multicellular organisms need to transmit materials and information Metabolic products have to be distributed throughout the body Information has to be transmitted to integrate activities Raw materials have to be distributed – diffusion is inadequate for larger organisms © 2017 American Society of Plant Biologists

Vascular plants have long-distance transport systems XYLEM Water moves from the soil to the atmosphere through the hollow dead cells of the xylem PHLOEM Photosyntheticallyproduced sugars (and other molecules) move from their source to sinks (nonphotosynthetic tissues) through the phloem © 2017 American Society of Plant Biologists

Water uptake and movement in vascular plants Water is pulled through the hollow xylem by tension developed at evaporative sites in the leaves In the leaf, water evaporates out of the xylem into the intracellular spaces, and then through the stomata into the atmosphere Stomate Endodermis Water moves from the soil, into the outer layers of the root , then into the vascular cylinder and xylem Vascular cylinder Outer root layer © 2017 American Society of Plant Biologists

Water movement in the xylem is driven by evaporation Ψw= -15 to -100 MPa Simple model: Hollow tube that water evaporates through Better model: Hollow tube with a selectivity filter in the roots and a flow regulator at the top Guard cells Ψw= -0. 2 MPa Casparian strip of the endodermis Adapted from Lucas, W. J. et al. (2013). The plant vascular system: Evolution, development and functions. J. Integr. Plant Biol. 55: 294 -388. © 2017 American Society of Plant Biologists

The root endodermis acts as a selectivity filter Na+ Na+ The endodermis produces a water-impermeable layer, the Casparian strip, that provides selectivity Na+ Na+ Photo credit Michael Clayton © 2017 American Society of Plant Biologists

Waxy cuticles prevent water loss; regulated pores allow it OPEN Most plant aerial surfaces are covered by a waxy cuticle. Pores called stomata, usually covered by pairs of guard cells, permit transpiration Guard cells change their volume to open and close the pore. Guard cells are sensitive to the atmospheric conditions and the plant’s needs for gas exchange and water conservation CLOSED © 2017 American Society of Plant Biologists

Transport in the phloem From source to sink 1. Sugars are loaded into the phloem by active transport CC Sugars 2. Water moves in by osmosis 4. Sugars are released into the sink tissues SE 3. The fluid moves through the sieve elements under pressure, by bulk flow (like water in a hose) 5. Water follows by osmosis Adapted from Lucas, W. J. et al. (2013). The plant vascular system: Evolution, development and functions. J. Integr. Plant Biol. 55: 294 -388. © 2017 American Society of Plant Biologists

Vascular tissues are essential conduits for information flow Some signals move in the xylem. Signals from droughtstressed roots cause guard cells to close In animals, signals moving through the nervous and circulatory systems convey information Other xylem-borne signals convey information about nutrient availability and soil-microbes Reprinted from Schachtman, D. P. and Goodger, J. Q. D. (2008). Chemical root to shoot signaling under drought. Trends Plant Sci. 13: 281 -287 with permission from Elsevier; see also Christmann, A. , Grill, E. and Huang, J. (2013). Hydraulic signals in long-distance signaling. Curr. Opin. Plant Biol. 16: 293 -300. © 2017 American Society of Plant Biologists

Plants can survive across most of the earth Arctic Mountain Antarctic Desert Photo credits: Hannes Grobe, AWI; Gnomefilliere; Liam Quinn; Florence Devouard © 2017 American Society of Plant Biologists

Obtaining and retaining water is a challenge for terrestrial plants Freshwater green algae easily take up water from their aquatic environment The water potential of air and soil is usually lower than that of the plant cells. How do terrestrial plants survive? © 2017 American Society of Plant Biologists

Desiccation tolerance or avoidance, drought evasion or tolerance Most bryophytes can tolerate desiccation (drying out extensively) Most tracheophytes cannot tolerate desiccation – they die Some desert plants evade drought. They survive the dry season as seeds, sprouting and flowering in a brief period of rain Some desert plants tolerate dry conditions through adaptations such as deep roots, C 4 photosynthesis, and tiny or absent leaves Photo credits: Mary Williams; Amrum; Scott Bauer; James Henderson, Golden Delight Honey, Bugwood. org © 2017 American Society of Plant Biologists

SUMMARY Like animals, plants need energy, water, and the ability to tolerate environmental challenges Plants have endosymbiotic photosynthetic organelles that let them produce chemical energy from light The two groups of plants, bryophytes and tracheophytes, differ in size, how they move materials, and how they deal with desiccation Image credit: Forest & Kim Starr, Starr Environmental, Bugwood. org © 2017 American Society of Plant Biologists